Global Navigation Satellite Systems (GNSS) is the commonly accepted term for all weather navigation and positioning systems using line of sight radio from orbiting satellites. The Global Positioning System (GPS), which is funded and controlled by the U.S. Department of Defence, is likely the most well known type of GNSS, however, GLONASS which is founded and controlled by Russia, as well as, Europe's GALILEO and China's Beidou, which are currently under development, are also examples of GNSS.
The concept of GNSS positioning is based on the simultaneously ranging to a minimum of four GNSS satellites from a receiver. With known satellite coordinates, the four dimensional coordinates of the receiver position, which include three spatial coordinates and a receiver clock offset, can be determined.
The receiver can approximate a distance to each satellite the receiver is receiving a signal from. The received signals indicates the position of the satellite transmitting the signal and by approximating the time that was required for the transmitted signal to travel from the satellite to the receiver antenna, a distance to the satellite can be approximated.
For a typical GNSS system, the signals transmitted by the satellites allow two types of measurements to be made. A first portion of the signal, the code portion, allows a code pseudorange measurement to be determined and another portion of the signal, the carrier portion, allows a carrier phase measurement to be determined. For example, in GPS systems each transmitted signal has a code portion (C/A code or P code) and a carrier portion.
To approximate a distance to a satellite using the code portion of the signal to determine a code pseudorange measurement, the code portion of the signal transmitted by the satellite is extracted and compared to an identical signal generated locally on the receiver. The time shift necessary to align the received signal with the internally generated signal is then used to approximate the travel time for the transmitted signal to travel between the satellite and the receiver. By using the travel time and the speed of light, the code pseudorange, which is an approximate of the distance to the satellite, can be determined. The code pseudorange measurement is an unambiguous approximation of distance to the satellite transmitting the signal. However, the code portion of the transmitted signal generally has wavelengths in the tens of meters to hundreds of meters in length, so this approximation of the distance to the satellite, by itself yields relatively poor position accuracy.
The second type of measurement, a carrier phase measurement, can be used to obtain a more accurate distance to the satellite transmitting the signals. The carrier portion of the transmitted signal can be extracted and used to obtain the carrier phase measurement. The carrier phase measurement is the phase difference between the carrier portion of the received satellite signal and a receiver-generated carrier portion of the signal. The carrier portion of the signal generally has wavelengths in the tens of centimeters, which can allow better position accuracy than the code portion of the signal with its longer wavelengths. However, the carrier portion of the signal consists of a repeated waveform and one carrier wave cannot be distinguished from another by the receiver. Initially, the receiver can only determine how far out of phase the received carrier portion of the signal is from the internally generated carrier portion of the signal and therefore can only determine a distance to the satellite transmitting the signal that consists of a distance that is less than a wavelength of the carrier portion of the signal plus an unknown amount of carrier waveforms. This is referred to as ambiguity or integer ambiguity because there is unknown number (integer) of carrier wavelengths plus the determined fractional distance to the satellite transmitting the signal. However, because of the small wavelength of the carrier portion of the signal relative to the wavelengths of the code portion of the signal, the receiver can determine the difference in phase shift between the received carrier portion of the signal and the internally generated carrier signal to a small fraction of the wavelength of the carrier signal, making it potentially much more accurate than using the code portion of the signal to determine differences.
However, even using the carrier portion of the signal to determine differences is not completely accurate because the signals being transmitted between the satellites and the receiver are subjected to numerous errors, such as satellite orbit, satellite clock error, atmosphere delay, environmental affections and the like. Augmentation systems have been developed to mitigate these errors. These systems determine errors and provide corrections for correcting code pseudorange measurements and carrier phase measurements determined by a receiver. These augmentation systems typically use one or more receivers located at precisely known coordinates to observe errors in received signals from the satellites in a GNSS system and calculate corrections for these observed errors. The augmentation systems can be ground based or satellite based and some freely provide the corrections whereas others require a subscription and the payment of subscription fees in order to receive and use the corrections. These augmentation systems include freely available augmentation systems such as the Wide Area Augmentation System (WAAS) covering North America, the European Geostationary Navigation Overlay System (EGNOS) covering Europe and the Multifunctional Transport Satellite Space bases Augmentation System (MSAS) covering East Asia, and privately owned systems, such as OmniStar™ and StarFire™. Typically, the publicly available augmentation systems provide the corrections for free, but only provide code corrections. The privately owned augmentation systems, typically provide carrier phase corrections, as well, but usually require the user to have a subscription to receive and use the corrections.
The code corrections received from the publicly available augmentation systems are applied to the code pseudoranges determined by a receiver and the corrected code pseudorange is then smoothed using the carrier phase. However, carrier phase smoothing fundamentally remains a code pseudorange based positioning method where the code pseudorange is the dominant measurement.